1. Field of the Invention
The present invention relates generally to a pressure variation monitoring device for monitoring small pressure variation and in particular to a non-intrusive physical measuring device which is suitable for detection of abdominal pressure variation of a pregnant woman such as that caused by uterine contraction.
2. Description of the Prior Art
With the development of technology, most of the modem products, such as electronic devices, are getting smaller in size and less heavy in weight. For uterine contraction monitors, developments have been made to change the uterine contraction monitors from a complex, expensive and non-portable apparatus into a small, light-weighted and thus portable device which is particularly suitable for long-term ambulatory monitoring of the abdominal pressure variation or uterine contraction of a pregnant woman. Such conventional portable pressure variation detection/monitoring devices, although effective in detecting considerable pressure variation, yet is incapable to detect small or tiny pressure variation and may also be incapable to provided a stable and reliable operation.
FIG. 1 of the attached drawings schematically shows the principle of a conventional pressure variation monitoring device, which comprises a resistive detection element, such as a strain gauge 101, to detect the pressure variation. A pressure variation causes the strain gauge 101 to deform which in turn results in a change in the resistance of the strain gauge. Such a change in resistance leads in a corresponding change in voltage and thus may be detected, for example by means of a Wheatstone bridge as shown in FIG. 1 and converted by means of an analog-to-digital converter to provide digital readings of the pressure variation.
However, in detecting a small pressure variation, the error of the detection result of the strain gauge type pressure variation monitoring device is not acceptable for the measure is done on the basis of the change of the resistance caused by the deformation of the strain gauge. Since the resistance of the strain gauge may also vary with the change of environmental temperature, the precision of detection is significantly affected by the environmental temperature. Moreover, the location where the external pressure is applied to the strain gauge is also a factor that affects the deformation of the strain gauge and thus the detection result obtained therefrom.
To overcome such a drawback, a different pressure variation monitoring device has been proposed which is shown in FIG. 2 of the attached drawings. The device shown in FIG. 2 comprises a linear variable differential transformer (LVDT) circuit which has been widely used in detection of small pressure variation. In FIG. 2, the arrow represents the direction in which a displacement of a magnetic bar 21 is made by means of the pressure variation detected and such a displacement of the magnetic bar 21 induces output voltages Vce and Vde between output terminals c and e and output terminals d and e. The LVDT device shown in FIG. 2, although effective in providing a more stable and reliable reading of the pressure variation, yet it is not suitable to serve as a portable device for it is operated by alternate current which is usually supplied by an electrical main.
Thus it is desirable to provide a pressure variation monitoring device which overcomes the stable reading problem and is suitable for ambulatory monitoring purpose.